Gelatin has been used extensively in a variety of imaging elements as the binder because of its many unique and advantageous properties. For example, its property of water swellability allows processing chemistry to be carried out to form silver halide-based photographic images, and its hydrophilic nature allows gelatin to function as an ink-receiver in ink-jet recording media. However, due to this same property, imaging elements with exposed gelatin-containing materials, no matter if they are formed on transparent or reflective media, have to be handled with extreme care so as not to be in contact with any aqueous solutions that may damage the images. Accidental spillage of common household solutions such as coffee, punch, or even plain water can damage imaging elements such as ink-jet, electrophotographic, or photographic prints.
There have been attempts over the years to provide protective layers for gelatin-based photographic systems that will protect the images from damage by water or aqueous solutions. U.S. Pat. No. 2,173,480 describes a method of applying a colloidal suspension to moist film as the last step of photographic processing before drying. A number of patents describe methods of solvent coating a protective layer on the image after photographic processing is completed and are described, for example, in U.S. Pat. Nos. 2,259,009, 2,331,746, 2,798,004, 3,113,867, 3,190,197, 3,415,670 and 3,733,293. More recently, U.S. Pat. No. 5,376,434 describes a protective layer formed on a photographic print by coating and drying a latex on a gelatin-containing layer bearing an image. The latex is a resin having a glass transition temperature of from 30.degree. C. to 70.degree. C. Another type of protective coating involves the application of UV-polymerizable monomers and oligomers on a processed image followed by radiation exposure to form crosslinked protective layer, which is described in U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. A drawback for both the solvent coating method and for the radiation cure method is the health and environmental concern of those chemicals or radiation to the coating operator. Another drawback is that the photographic materials need to be coated after the processing step. Thus, the processing equipment needs to be modified and the personnel running the processing operation need to be trained to apply the protective coating.
Various lamination techniques are known and practiced in the trade. U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating a polymeric sheet film, as a protective layer, on a processed image. Protective coatings that need to be applied to the image after it is formed, several of which were mentioned above, add a significant cost to the final imaged product. Therefore, a number of patents have been directed to water-resistant protective coatings that can be applied to a photographic element prior to development. For example, U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the light-sensitive layer, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing. The overcoat as described is coated as a suspension in an organic solvent, and thus is not desirable for large-scale application. More recently, U.S. Pat. No. 5,853,926 to Bohan et al. discloses a protective coating for a photographic element, involving the application of an aqueous coating comprising polymer particles and a soft polymer latex binder. The patent lists many polymers including polyurethane. This coating allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. Again, however, the hydrophobic polymer particles must be fused to form a protective coating that is continuous and water-impermeable. Unfortunately, the physical integrity of this coating is not good, and the large particles tend to fall off the coating before fusing.
Protective coatings containing gelatin-grafted polyurethanes using pendant carboxylic acid groups on the polyurethanes for grafting, further in combination with polyvinyl alcohols, have been disclosed for use as overcoats on the non-emulsion side of photographic elements. See, for example, U.S. Pat. Nos. 5,910,401 and 5,846,699. U.S. Pat. No. 5,846,699 discloses a mixture of a polyurethane and a carboxylic acid containing polymer having an acid number of 60 to 260, to provide resistance to abrasion and the like, for example on the backing of a photographic element.
The ability to provide the desired property of post-process water/stain resistance of an imaged photographic element, at the point of manufacture of the photographic element, is a highly desired feature. However, in order to accomplish this feature, the desired photographic element should be permeable to aqueous solutions during the processing step, but after processing achieve water resistance and even water impermeability for at least some time after contact with water. Commonly assigned U.S. Ser. No. 09/235,436 now U.S. Pat. No. 6,077,648, discloses the use of a processing solution permeable overcoat that is composed of a urethane-vinyl copolymer having acid functionalities. Commonly assigned U.S. Ser. No. 09/235,437 and U.S. Ser. No. 09/448,213 now U.S. Pat. No. 6,194,130, disclose the use of a second polymer such as water-soluble (non-cross-linked) gelatin or polyvinyl alcohol to reduce such defects and disadvantages.
Protective coats on recording media have similar challenges. Although a developing solution for that includes an oxidizable silver halide developing agent is not involved, the recording system often involves the application of a water soluble dye or pigment to a hydrophilic recording layer in the recording medium. After the application of the color material but not before, however, it may be desirable to render the surface of the medium water resistant. For example, European Patent Application 0,858,905A1 describes a porous, particulate outermost layer applied to ink-jet recording media, that is heat-fused after ink absorption to form a protective surface. Such porous surface layers are often mechanically weak and in particular can be prone to scratching damage or material removal during transport and handling prior to image formation and subsequent fusing, thus compromising the usefulness of their protective function.
U.S. Pat. No. 5,856,051 describes the use of hydrophobic particles with gelatin as the binder in an overcoat formulation. This invention demonstrated an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. The hydrophobic polymers exemplified in U.S. Pat. No. 5,856,051 include polyethylene having a melting temperature (Tm) of 55 to 200.degree. C., and therefore capable of forming a water-resistant layer by fusing the layer at a temperature higher than the Tm of the polymer after the sample has been processed to generate the image. The coating solution is aqueous and can be incorporated in the manufacturing coating operation without any equipment modification. The fusing step is simple and environmentally friendly to photofinishing laboratories. Since the particles are incorporated entirely within the uppermost layer, this approach does not suffer from a lack of mechanical strength and integrity during transport and handling prior to image formation and fusing. However, the scratch resistance of such an overcoat after fusing is a serious concern, since polyethylene is a very soft material.
Similarly, commonly assigned U.S. Ser. No. 09/353,939 describe the use of a polystyrene-based material, with gelatin as the binder, in an overcoat for a photographic element, which overcoat can be fused into a water resistant overcoat after photographic processing is accomplished to generate an image.
In the context of photographic development, there is apparently a limited selection of materials that can be used in the overcoat of a gelatin-based photographic product, which allows photographic image processing to be carried out for image formation and subsequently transforms into a water resistant layer by the means of heat and pressure. Therefore, there remains a need for, and it would be highly desirable to obtain, an overcoat applied to a photographic element before development that would not significantly reduce the rate of reaction of the developer with the underlying emulsions, but which would ultimately provide a water resistant and durable overcoat after the processing or developing step. There is likewise a need for an improved coating applied to a recording element, such as ink-jet media, before printing that would not significantly reduce the rate of absorption of an aqueous coloring solution into the underlying recording layers, but which would ultimately provide a water resistant and durable overcoat after the print or image is formed on the recording element. Finally, there is a need for an improved coating applied to a recording element, such as electrophotograhic media, that would provide a water-resistant coating after the usual fusing step.